vtec controlled by tach

I m doing a ls/vtec a 91 civic 4 dr ex with a 91 b18a1 with a b16a head i was told you can hook your vtec up to your tach instead of running a vtec ecu and switching wires around can anyone help me out

My friend did that. He had a 88 dx hatch, 98 vtec head, 87 dx tranny. Ghetto as it comes. but the vtec was hooked up to the shiftlight so whenever the shiftlight turned on the vtec engaged. Ghetto as hell but it worked. Like cyclone said buy the ecu there cheap for sohc vtecs anyway.